From Underwood's Pathology, 6th Ed, Simon S. Cross, Chp 9 - Acute Inflammation pg.168
The ultrastructural basis of increased vascular permeability was originally determined using an experimental model in which histamine, one of the chemical mediators of increased vascular permeability, was injected under the skin. This caused transient leakage of plasma proteins into the extravascular space. Electron microscopic examination of the venules and small veins during this period showed that gaps of 0.1 - 0.4 micrometres in diameter had appeared between endothelial cells. ... The endothelial cells are not damaged in this process. They contain contractile proteins such as actin, which, when stimulated by the chemical mediators of acute inflammation, cause contraction of the endothelial cells, pulling open the transient pores."